1. Field of the Invention
This invention relates to a process for producing cured products having superior properties such as initial conductivity, retention of conductivity, and low temperature moldability, which process comprises reducing copper in the cuprous and/or cupric state (copper in the compound state, copper in the oxidized state) in the presence of a metallic copper powder on a substrate to convert said copper to metallic copper, thereby forming a conductive connected unit of the metallic copper powder, and subjecting the resulting unit to cure molding using a resinous curable component.
2. Description of the Prior Art
Metals have superior electric conductivity, but suffer from the defect that because of their high melting points, they especially have inferior moldability at relatively low temperatures when compared with resins. Resins, on the other hand, have good moldability at relatively low temperatures, but since they generally have low conductivity as shown by their resistivity of 10.sup.11 ohms.cm or more, they cannot find applications in electrical fields which require a high level of conductivity, for example, as component parts of an electrical conductive circuit.
It would be necessary to consider the choice of metals in order to utilize both the conductivity of metals and the low temperature moldability of resins. Heretofore, a copper foil has been used as a material for a conductive circuit in printed circuit boards or the like and, in view of this, the development of a method which can afford a conductive cured product having good moldability at low temperatures using copper powder would be very useful in that the cured product obtained can be used in the form of paints, adhesives, or the like in various practical applications, for example, as members of a conductive circuit. Such a method, however, is unable to afford cured products which are satisfactory both in electric conductivity and its retention.
Furthermore, according to a conventional method in which a copper powder is merely mixed with a resin, the improvement of conductivity of the resin cannot at all be expected in whatever great amount the copper powder may be used. Our investigations have ascertained that this is because the peripheral edges of the copper powder particles are oxidized with the oxygen in air to form a coating of copper oxide which has poor conductivity.
We have also found that a cured product prepared, in an attempt to solve the above problem, by, for example, dissolving copper oxide with an acid such as hydrochloric acid to form a copper oxide-free copper powder (metallic copper powder) in accordance with a known method, and mixing the copper powder with a resin followed by curing, cannot retain its conductivity in air for long periods of time but gradually loses its conductivity, with the consequence that its conductivity is completely lost within a very short period of time of, say, about 1 day to 1 month. Therefore, cured products obtained by such a method using metallic copper powder have little or no ability to retain their initial conductivity.
Since good ability to retain conductivity is an absolute requirement in applications which require conductivity over long periods of time as in the case of members of electrical conductive circuits, the above method using metallic copper powder free from a copper oxide coating is not commercially feasible because of the defect of poor conductivity.
When a method is used in which a mixture of a metallic copper powder free from a copper oxide coating and a resin is cured and the cured product is treated with a treating agent for preventing oxidation of the surface of the metallic copper powder, such as an imidazole compound or benzotriazole, the oxidation of the surface of the metallic copper powder can be retarded to some extent, but the results are not satisfactory nor fully reliable with respect to the ability to retain conductivity. This means that a conductive cured product based on mere contact between metallic copper particles cannot possess sufficient reliability for practical purposes.